1. Field of the Invention
This invention relates to an optical wavelength converting device for converting a fundamental wave into its second harmonic, or the like. This invention particularly relates to an optical wavelength converting device having a periodic domain inversion structure. This invention also relates to a process for producing the optical wavelength converting device. This invention further relates to a solid laser for converting a produced laser beam into its second harmonic by the utilization of the optical wavelength converting device and radiating out the second harmonic.
2. Description of the Related Art
A technique, wherein a fundamental wave is converted into its second harmonic by the utilization of an optical wavelength converting device having a periodic domain inversion structure has been proposed by Bleombergen, et al. in Phys. Rev., Vol. 127, No. 6, 1918 (1962). The periodic domain inversion structure is provided with regions, in which spontaneous polarization (domain) of a ferroelectric substance having nonlinear optical effects is inverted periodically. With the proposed technique, phase matching between a fundamental wave and its second harmonic can be effected by setting such that a period Λ of the domain inversion regions may be integral multiples of a coherence length Λc, which may be represented by Formula (1) shown below.Λc=2π/{β(2ω)−2β(ω)}  (1)in which β(2ω) represents the propagation constant of the second harmonic, and β(ω) represents the propagation constant of the fundamental wave.
In cases where wavelength conversion is performed by using a bulk crystal of a nonlinear optical material, the wavelength at which the phase matching is effected is limited to a specific wavelength that is inherent to the crystal. However, with the proposed technique, the phase matching can be effected efficiently by selecting the period Λ of the domain inversion regions, which period satisfies Formula (1), with respect to an arbitrary wavelength.
One of techniques for forming the periodic domain inversion structure described above has been proposed in, for example, Japanese Unexamined Patent Publication No. 7(1995)-72521. With the proposed technique for forming the periodic domain inversion structure, after a periodic electrode in a predetermined pattern is formed on one surface of a single-polarized ferroelectric substance having nonlinear optical effects, an electric field is applied through corona charge across the ferroelectric substance by the utilization of the periodic electrode and a corona wire, which is located on the surface side of the ferroelectric substance opposite to the one surface of the ferroelectric substance, and regions of the ferroelectric substance which stand facing the periodic electrode are thereby set as local area limited domain inversion regions.
A different technique for forming the periodic domain inversion structure described above has been proposed in, for example, Japanese Unexamined Patent Publication No. 4(1992)-335620. With the proposed technique for forming the periodic domain inversion structure, an entire-area electrode is formed on a surface of a ferroelectric substance on the side opposite to a surface on which a periodic electrode in a predetermined pattern is formed, an electric field is applied across the ferroelectric substance by the utilization of the entire-area electrode and the periodic electrode, and local area limited domain inversion regions are thereby formed.
As a technique for forming the periodic electrode, a technique, wherein ridge regions having predetermined shapes in a predetermined pattern are formed on one surface of a ferroelectric substance, and electrode fingers of a periodic electrode are formed on the surfaces of the ridge regions, has been proposed in, for example, Japanese Unexamined Patent Publication No. 10(1998)-170966.
In cases where the periodic domain inversion structure is formed by the utilization of the periodic electrode in the manner described above, particularly as for a Z-cut ferroelectric substance plate, there is a strong possibility that, as the period of the periodic electrode is set to be short in order for a second harmonic, or the like, having a short wavelength to be generated, domain inversion regions, which are adjacent to each other and extend through the ferroelectric substance from the areas corresponding to electrode fingers of the periodic electrode, will become connected with each other.
The problems described above will be described hereinbelow with reference to FIG. 7. FIG. 7 is a graph showing results of evaluation of periodicity of various bulk-form periodic domain inversion structures, each of which is formed in LiNbO3 doped with MgO (hereinbelow referred to simply as MgO-LN) by the utilization of a periodic electrode having an electrode line width (i.e., the line width of each of the electrode fingers of the periodic electrode) A, the evaluation being made with respect to various different values of a period Λ of domain inversion regions and various different values of a duty ratio D (D=A/Λ). In FIG. 7, the “◯” mark indicates that the periodicity is good over a length of at least 1 mm. The “Δ” mark indicates that the periodicity is good only over a length of less than 1 mm or that the regions in which the periodicity is good occur sporadically. The “x” mark indicates that few regions in which the periodicity is good occur.
As shown in FIG. 7, in order for good periodicity of the periodic domain inversion structure to be obtained, it is efficient to set the duty ratio D at a small value, i.e., to set the electrode line width A at a small value. Also, in cases where the period Λ of the domain inversion regions is at most 7 μm, it is necessary for the duty ratio D to be set at a value of at most 0.15. In cases where the domain inversion length is approximately 1 mm, the duty ratio D should thus be set at a value of at most 0.15. In the cases of large areas (in cases where the domain inversion length is approximately 3 mm to 4 mm), such that the inversion periodicity may be enhanced, the duty ratio D should be set at a value smaller than the value of at most 0.15.
In cases where the periodic domain inversion structure is formed by the utilization of the periodic electrode, each of the domain inversion regions is formed over a region slightly wider than the region corresponding to the electrode line width A due to the spread of the electric field. Therefore, even if the duty ratio D is set at a value smaller than 0.15, the periodic domain inversion structure can be formed, in which the ratio between the width of each domain inversion region and the width of each non-inversion region is approximately equal to 1:1.
In view of the above circumstances, in cases where a second harmonic, or the like, having a short wavelength falling within, for example, the blue region or the ultraviolet region is to be generated, it is necessary for a periodic electrode having a markedly small electrode line width A to be formed. However, heretofore, it was difficult to form a periodic electrode having a markedly small electrode line width A. Particularly, with respect to the optical wavelength converting device in which the periodic domain inversion structure is formed in the bulk form in a crystal of a Z-cut plate of MgO-LN, an example in which a second harmonic having a wavelength falling within the wavelength region of at most 470 nm has not heretofore been reported. The term “periodic domain inversion structure in a bulk form in a crystal of a Z-cut plate” as used herein means the periodic domain inversion structure in which the domain inversion regions are formed over a range extending from a position in the vicinity of a +Z surface of the plate to a position in the vicinity of a −Z surface of the plate.
In cases where a second harmonic having a wavelength falling within the wavelength region of at most 470 nm is to be generated with the aforesaid type of the optical wavelength converting device, if the electrode line width A of the periodic electrode employed for the formation of the periodic domain inversion structure is set at a value of at most 0.3 μm, a periodic domain inversion structure reliably having good periodicity over a wide area can be formed.
As techniques for forming a periodic electrode having a small electrode line width A, an EB drawing technique, an FIB deposition technique, and the like, have heretofore been known. However, the conventional techniques for forming a periodic electrode having a small electrode line width A are not appropriate for large-area patterning and have a low throughput and a productivity markedly lower than the level of productivity required for mass production.
As a technique capable of coping with large-area patterning, a technique utilizing a contraction exposure apparatus has heretofore been known. However, the technique utilizing the contraction exposure apparatus has the draw-backs in that the cost of the contraction exposure apparatus is markedly high and it is difficult to obtain an electrode line width A shorter than the wavelength of exposure light.